1. Field of the Invention
This invention relates to methods and catalysts to generate a hydrogen-rich gas from gas mixtures containing carbon monoxide and water, such as water-containing syngas mixtures. More particularly, the invention includes methods using platinum-based catalysts which contain alkali or alkaline-earth metals. The catalysts may be supported on a variety of catalyst support materials. Catalysts of the invention exhibit both high activity and selectivity to hydrogen generation and carbon monoxide oxidation.
2. Discussion of the Related Art
Numerous chemical and energy-producing processes require a hydrogen-rich composition (e.g. feed stream.) A hydrogen-rich feed stream is typically combined with other reactants to carry out various processes. Nitrogen fixation processes, for example, produce ammonia by reacting feed streams containing hydrogen and nitrogen under high pressures and temperatures in the presence of a catalyst. In other processes, the hydrogen-rich feed stream should not contain components detrimental to the process. Fuel cells such as polymer electrode membrane (PEM) fuel cells, produce energy from a hydrogen-rich feed stream. PEM fuel cells typically operate with a feed stream gas inlet temperature of less than 450° C. Carbon monoxide is excluded from the feed stream to the extent possible to prevent poisoning of the electrode catalyst, which is typically a platinum-containing catalyst. See U.S. Pat. No. 6,299,995.
One route for producing a hydrogen-rich gas is hydrocarbon steam reforming. In a hydrocarbon steam reforming process steam is reacted with a hydrocarbon fuel, such as methane, iso-octane, toluene, etc., to produce hydrogen gas and carbon dioxide. The reaction, shown below with methane (CH4), is strongly endothermic; it requires a significant amount of heat.CH4+2H2O→4H2+CO2 In the petrochemical industry, hydrocarbon steam reforming of natural gas is typically performed at temperatures in excess of 900° C. Even for catalyst assisted hydrocarbon steam reforming the temperature requirement is often still above 700° C. See, for example, U.S. Pat. No. 6,303,098. Steam reforming of hydrocarbons, such as methane, using nickel- and gold-containing catalysts and temperatures greater than 450° C. is described in U.S. Pat. No. 5,997,835. The catalyzed process forms a hydrogen-rich gas, with depressed carbon formation.
One example of effective hydrocarbon steam reforming catalysts is the Sinfelt compositions which are composed of Pt, a Group 11 metal, and a Group 8-10 metal. Group 11 metals include Cu, Ag and Au while Group 8-10 metals include the other noble metals. These catalyst formulations are well known in the promotion of hydrogenation, hydrogenolysis, hydrocracking, dealkylation of aromatics, and naphtha reforming processes. See, for example, U.S. Pat. Nos. 3,567,625 and 3,953,368. The application of catalysts based on the Sinfelt model to the water gas shift (“WGS”) reaction, in particular at conditions suitable for lower temperature WGS applications such as PEM fuel cells, has not been previously reported.
Purified hydrogen-containing feed streams have also been produced by filtering the gas mixture produced by hydrocarbon steam reformation through hydrogen-permeable and hydrogen-selective membranes. See, for example, U.S. Pat. No. 6,221,117. Such approaches suffer from drawbacks due to the complexity of the system and slow flow rates through the membranes.
Another method of producing a hydrogen-rich gas such as a feed stream starts with a gas mixture containing hydrogen and carbon monoxide and with the absence of any substantial amount of water. For instance, this may be the product of reforming a hydrocarbon or an alcohol, and selectively removes the carbon monoxide from that gas mixture. The carbon monoxide can be removed by absorption of the carbon monoxide and/or by its oxidation to carbon dioxide. Such a process utilizing a ruthenium based catalyst to remove and oxidize the carbon monoxide is disclosed in U.S. Pat. No. 6,190,430.
The WGS reaction is another mechanism for producing a hydrogen-rich gas but from water (steam) and carbon monoxide. An equilibrium process, the water gas shift reaction, shown below, converts water and carbon monoxide to hydrogen and carbon dioxide, and vice versa.H2O+COH2+CO2 Various catalysts have been developed to catalyze the WGS reaction. These catalysts are typically intended for use at temperatures greater than 450° C. and/or pressures above 1 bar. For instance, U.S. Pat. No. 5,030,440 relates to a palladium and platinum-containing catalyst formulation for catalyzing the shift reaction at 550° C. to 650° C. See also U.S. Pat. No. 5,830,425 for an iron/copper based catalyst formulation.
Catalytic conversion of water and carbon monoxide under water gas shift reaction conditions has been used to produce hydrogen-rich and carbon monoxide-poor gas mixtures. Existing WGS catalysts, however, do not exhibit sufficient activity at a given temperature to reach or even closely approach thermodynamic equilibrium concentrations of hydrogen and carbon monoxide such that the product gas may subsequently be used as a hydrogen feed stream. Specifically, existing catalyst formulations are not sufficiently active at low temperatures, that is, below about 450° C. See U.S. Pat. No. 5,030,440.
Platinum (Pt) is a well-known catalyst for both hydrocarbon steam reforming and water gas shift reactions. Under typical hydrocarbon steam reforming conditions, high temperature (above 850° C.) and high pressure (greater than 10 bar), the WGS reaction may occur post-reforming over the hydrocarbon steam reforming catalyst due to the high temperature and generally unselective catalyst compositions. See, for instance, U.S. Pat. Nos. 6,254,807; 5,368,835; 5,134,109 and 5,030,440 for a variety of catalyst compositions and reaction conditions under which the water gas shift reaction may occur post-reforming.
Metals such as cobalt (Co), ruthenium (Ru), palladium (Pd), rhodium (Rh) and nickel (Ni) have also been used as WGS catalysts but are normally too active for the selective WGS reaction and cause methanation of CO to CH4 under typical reaction conditions. In other words, the hydrogen produced by the water gas shift reaction is consumed as it reacts with the CO present in the presence of such catalysts to yield methane. This methanation reaction activity has limited the utility of metals such as Co, Ru, Pd, Rh and Ni as water gas shift catalysts.
A need exists, therefore, for a method to produce a hydrogen-rich syngas, and catalysts which are highly active and highly selective for both hydrogen generation and carbon monoxide oxidation at moderate temperatures (e.g. below about 450° C.) to provide a hydrogen-rich syngas from a gas mixture containing hydrogen and carbon monoxide.